Today, most projection displays are only capable of projecting 2-D images. Stereo 3D displays are useful for many applications. They provide additional depth information and allow viewers to extract information from complex data faster and more accurately. In addition, they create immersive environments that are very useful for visual simulations, 3D gaming and 3D movies.
In order to display stereo 3D images, a stereoscopic 3D projection display must be able to show at least two slightly different left-eye and right-eye images to be seen by the viewer's left and right eyes. There are several approaches to display three-dimensional stereoscopic images or videos by projection means. These include auto-stereoscopic displays which are limited to a single or a few viewers who do not need to wear 3D glasses; and projection displays where viewers wear active or passive 3D glasses. For multi-viewer and large screen applications, 3D stereoscopic projection displays that require viewers to wear 3D glasses are more suitable because they do not limit viewing positions or the number of viewers. Detailed description of each approach can be found in open literature.
There are two types of stereoscopic 3D projection displays using 3D glasses: passive and active. In passive stereoscopic 3D displays with glasses, the left- and right-eye images are displayed with light in two different polarizations or in two different sets of colors. They normally require two projectors: one to project the left-eye images in one polarization or set of colors, the other to project the right-eye images in the orthogonal polarization or a different set of colors. Polarizing glasses or color filter glasses are relatively inexpensive and suitable for large audiences such as in a meeting room or a 3D cinema. However, dual projectors are bulky, expensive, and difficult to align. In addition, they are not light efficient, only 12-30% of the light is used in 3D when compared to 100% for displaying 2D images. Single projector versions of passive stereoscopic display systems also exist, such as projection displays using Z-Screens. In this case, the left- and right-eye images are displayed time-sequentially, further reducing light efficiency because the left- and right-eye images are displayed at most only half of the time. Typical light efficiency is only about 12% and thus much higher power lamps must be used. In addition, 3D projectors using Z-screens must operate at faster frame rates. Only expensive 3-chip DLP projectors or cathode ray tubes (CRT) have such capability.
In 3D displays with active glasses, the left- and right-eye images are displayed time-sequentially and viewers wear LCD shutter glasses that are synchronized with the appearance of the correct eye image. Only about 16% of light from the projector is used for 3D. Active glasses require power and a wired/wireless link to the projector. In addition, they can generally only work with fast refresh rate CRTs or expensive 3-chip DLP projectors.
In stereoscopic 3D projection displays using passive filter glasses, the left- and right-eye images are projected in spectrally separated sets of primary colors, for example, R1, G1, B1, and R2, G2, B2, respectively. Each set of primary colors can form full color images although the color gamut seen by each eye can be slightly different. This difference can be corrected for by the projection system. Using two different sets of primary colors for stereoscopic 3D projection displays has been disclosed. U.S. Pat. No. 7,001,021 discloses an arrangement which combines the left and right-eye color images having different sets of primary colors with a dichroic filter or prism. For projection displays, the projection optical system usually requires reasonably large aperture in order to use as much illumination light as possible, for example, f/2.4 optics with divergent angle of ±12° in air. It is well known that dichroic filters are very sensitive to angles of incidence and polarization states of the incident light and their performance changes significantly with angles and polarization. Since colors R1 and R2 are usually very close to each other spectrally, so are colors G1, G2, and B1,B2. It is very difficult or impractical to combine the colors effectively with a dichroic filter or prism in order to project both sets of images simultaneously through a single lens projector. In order to make this type of projection system work, the divergence angle would have to be reduced significantly and this makes it very light inefficient and impractical for use in stereoscopic displays.